This invention relates to a process for producing diborane.
Diborane (B.sub.2 H.sub.6) is a known compound which is manufactured commercially for a number of uses. A massive amount of research has been carried out into methods for its synthesis, as shown in the following review articles:
I. R. T. Holzman, "Production of the Boranes and Related Research", Academic Press (1967), Chapter III; PA1 II. R. M. Adams, "Boron, Metallo-Boron Compounds and Boranes", Interscience Publishers (1964), pages 559-571, plus list of references on Pages 670-690; and PA1 III. R. M. Adams, "Preparation of Diborane", in "Borax to Boranes", Advances in Chemistry Series No. 32, Americal Chemical Society (1961), Pages 60-68.
Of the numerous possible methods for producing diborane described in the above review articles, one type of reaction which proceeds almost quantitatively and which is used commercially to produce diborane is the reaction of an alkali metal borohydride with boron trifluoride in the presence of a solvent, the solvent normally being an ether. Although as stated above, this reaction does proceed almost quantitatively, it has the very serious disadvantage of using the highly inflammable ether solvent, which may vary from diethyl ether to polyethers such as diglyme. Although those skilled in the art of chemical engineering are of course familiar with safety precautions necessary to work with highly inflammable ether solutions on an industrial scale, the need for a highly inflammable solvent in a diborane-producing reaction is especially disadvantageous because diborane itself is spontaneously inflammable in air. Thus, the slightest leak of diborane from the closed vessel in which it is produced into the atmosphere carries the risk that the resultant spontaneous ignition of the leaking diborane will cause a fire which will spread to the large quantities of highly volatile, highly inflammable ether solvent being used. The extreme safety precautions necessary to avoid the potentially catastrophic consequences of a minor leak of diborane when that compound is being produced on an industrial scale are a significant factor in the high cost of diborane; even when purchased in ton quantities, diborane costs in excess of $200 per pound ($420 per kilogram). Furthermore, the cost of the ether solvent itself is a major factor in the cost of diborane, since the polyethers such as diglyme which are preferred for ease of separation of the diborane therefrom are expensive.
Accordingly, there has long been a need in the art for a process for producing diborane which does not involve the use of inflammable solvents. However, those skilled in the art have hitherto believed that it is not possible to react alkali metal borohydrides with boron trihalides except in the presence of a solvent. Extensive investigations of the mechanism of the reaction between alkali metal borohydrides and boron trihalides have shown that the yields are controlled by kinetic rather than thermodynamic factors and that the solubilities of the reaction in a single phase are critical. Furthermore, it has been shown that in such a reaction the boron trihalide reacts in the form of an etherate complex and that the formation of diborane is preceded by an intermediate stage in which one or more alkoxy groups derived from the ether become attached to the boron (see Reference I above, Pages 35-36 and Reference II above, Pages 563-564). In other words, the ether is acting not merely as a solvent but also as an essential reagent in the formation of an alkoxyborane which is formed as an intermediate prior to the production of diborane itself. Accordingly, those skilled in the art have hitherto had every reason to believe that it is not possible to react alkali metal borohydrides with boron trihalides in the absence of a solvent and have been compelled to continue using the hazardous "wet" processes with the expensive ether solvents.